1. Field of the Invention
The present invention relates to a silicate phosphor which exhibits high-luminance blue light emission by excitation by light in a visible region from near-ultraviolet, and method of producing the same.
2. Description of the Related Art
A white LED is produced by combining a blue or near-ultraviolet LED (LD) and a phosphor. Because of low emission efficiency, white LEDs have been developed mainly for the purpose of use as a backlight for portable telephones and the like. In recent years, however, with an increase in emission efficiency, they attract attention as next-generation illumination.
Systems of configuring a white LED including a system of combining a blue LED and a yellow phosphor, a system of combining a near-ultraviolet LED and blue, green, and red phosphors have been suggested.
In a near-ultraviolet LED excitation system, a blue-based phosphor is required in which excitation occurs in the neighborhood of 400 nm, which is an emission wavelength of the LED. (Ca, Sr)5(PO4)3Cl:Eu, BaMgAl10O17:Eu (BAM), and the like, which have been used as conventional fluorescent lamp, have been improved and used.
For example, Tsunemasa Taguchi, “All about white LED lighting technology”, Kogyo Chosakai Publishing Co., Ltd., p. 110 describes an excitation spectrum and an emission spectrum of BAM improved for near-ultraviolet LED excitation.
As one of phosphors that emit blue light with near-ultraviolet excitation, BaZrSi3O9:Eu has been known. G. Blasse, A. Bril, Journal of Solid State Chemistry, 1970, vol. 2, p. 105 to 108 describes Ba0.99ZrSi3O9:0.01Eu.
Also, Japanese Examined Patent Publication No. 48-38550 discloses (Ba,Sr)0.99ZrSi3O9:0.01Eu, and Japanese Patent Application Laid-Open No. 2008-63550 discloses that as a composition of (Ba(1−x−y)SrxEuy) (Sn1−zZrz)Si3O9, Sr substitutes for a Ba site and Sn substitutes for a Zr site, thereby increasing emission intensity in near-ultraviolet excitation.
However, when the excitation spectrum of BAM described in Tsunemasa Taguchi, “All about white LED lighting technology”, Kogyo Chosakai Publishing Co., Ltd., p. 110 is observed, it shows that the excitation intensity is abruptly decreased in the neighborhood of 400 nm, with 380 nm as a peak, and that since the excitation intensity corresponds to emission intensity at that excitation wavelength, this indicates that a change of emission intensity due to a change of an excitation wavelength is large (from analogy, it can be thought from a graph that a change of emission intensity in 380 nm to 420 nm is in the neighborhood of 60%).
When such an abrupt change of emission intensity occurs with respect to the excitation wavelength of the phosphor, fluctuations in blue-light emission intensity due to fluctuations in emission wavelength of an ultraviolet LED as an excitation source are increased, which leads to fluctuations in hue and emission intensity of the white LED. Therefore, this is not preferable.
As for Ba0.99ZrSi3O9:0.01Eu described in G. Blasse, A. Bril, Journal of Solid State Chemistry, 1970, vol. 2, p. 105 to 108 and Japanese Examined Patent Publication No. 48-38550, when its excitation spectrum (refer to Example 2 of FIG. 4(b); curve 6 of FIG. 3 in Japanese Examined Patent Publication No. 48-38550) is observed, it is evident that excitation intensity is abruptly decreased in a near-ultraviolet region and the emission intensity near an excitation wavelength of 400 nm is low (From the excitation spectrum of Japanese Examined Patent Publication No. 48-38550, by analogy, a change of emission intensity at 380 nm to 420 nm is in the neighborhood of 35% and the emission intensity at the excitation wavelength of 300 nm is in the neighborhood of 35% of emission intensity at an excitation wavelength of 400 nm).
Also, the phosphor is formed by a solid phase reaction in which a mixture of BaCO3, ZrO2, SiO2, and Eu2O3 are subjected to a heat treatment in nitrogen-hydrogen mixed gas. In the case of this solid phase reaction, it is difficult to obtain a phosphor with high main phase purity and with Eu uniformly dispersed therein. To make the solid phase reaction sufficiently progress, a heat treatment or repeated baking at high temperatures for a long hours is performed in general. However, such a heat treatment for a long hours and a heat treatment repeatedly performed are not preferable in an industrial point of view. Additionally, since both require crushing, there is a problem of decreasing luminance due to damage.
Moreover, regarding Ba0.98ZrSi3O9:0.02Eu disclosed as a comparative example of Japanese Patent Application Laid-Open No. 2008-63550, its excitation spectrum in the neighborhood of 400 nm is not disclosed. Furthermore, its emission intensity are represented by relative values, and it is thus difficult to compare absolute values. However, since Japanese Patent Application Laid-Open No. 2008-63550 has the producing method approximately similar to the producing method disclosed in G. Blasse, A. Bril, Journal of Solid State Chemistry, 1970, vol. 2, p. 105 to 108 (solid phase method), it can be said in Japanese Patent Application Laid-Open No. 2008-63550 that a change of emission intensity near an excitation wavelength of 400 nm is large and the emission intensity is insufficient. Also, in an embodiment of Japanese Patent Application Laid-Open No. 2008-63550, by Sr substitution for a Ba site, for example, relative luminance with excitation of 365 nm is increased a little over 1.9 times as large as Ba0.98ZrSi3O9:0.02Eu used as the comparative example. However, as described above, Ba0.98ZrSi3O9:0.02Eu itself has low intensity, and it cannot be said that the phosphor has practically sufficient luminance.
Under these circumstances, an object of the present invention is to provide a blue phosphor which is excited at a wavelength in the neighborhood of 400 nm, which is an emission wavelength of a near-ultraviolet LED, to emit high-intensity light and has a small change of emission intensity due to a change of an excitation wavelength, and a method of easily obtaining the phosphor.